1. Field
The present disclosure relates to a magnetic tunnel junction (MTJ) cell and a memory device including the same, and more particularly, to a thermally stable perpendicular magnetic anisotropy (PMA) MTJ cell and a memory device including the same.
2. Description of the Related Art
For example, a magnetic memory device, such as a Magnetic Random Access Memory (MRAM), is a memory device configured for storing data using a resistance change in an MTJ cell, and because the magnetic memory device is nonvolatile, operates at a high speed, and has high durability, lots of research into a magnetic memory device has been conducted as one of the next generation of nonvolatile memory devices.
In general, a resistance of an MTJ cell varies according to a magnetization direction of a free layer. For example, when the magnetization direction of the free layer is the same as a magnetization direction of a pinned layer, the MTJ cell may have a low resistance value. When the magnetization direction of the free layer is opposite to the magnetization direction of the pinned layer, the MTJ cell may have a high resistance value. When this characteristic is used for a memory device, for example, the low resistance value of the MTJ cell may correspond to data ‘0’, and the high resistance value of the MTJ cell may correspond to data ‘1’.
To maintain data stored in the MTJ cell without being damaged for a long time, the MTJ cell may have thermal stability. Commonly, the thermal stability of the MTJ cell is related to an energy barrier Eb of the free layer. For example, when the free layer is a magnetic anisotropy material layer and, if Eb/kT (k: Boltzmann constant, T: absolute temperature) is about 60, then it is known that the thermal stability of the MTJ cell is guaranteed for about 10 years. Although a little difference exists according to the thickness of the free layer, when a feature size of the MTJ cell is 40 nm or above, Eb/kT is about 60, which indicates that it is difficult to decrease the feature size of the MTJ cell to less than 40 nm. Accordingly, it is not easy to increase the recording density of an MRAM to a certain level or above. In addition, although the feature size of the MTJ cell may decrease when the thickness of the free layer increases, the increase in the thickness of the free layer may make it difficult to record data.
Accordingly, various attempts to decrease the feature size of the MTJ cell have been made. For example, there is a method of forming the MTJ cell using a perpendicular magnetic anisotropy (PMA) material. In general, because an energy barrier of the free layer increases as the magnetic anisotropy energy of the PMA material increases, the feature size of the MTJ cell may decrease by using a PMA material having high magnetic anisotropy energy. However, because most PMA materials having high magnetic anisotropy energy have a low magnetoresistance (MR) ratio, the PMA materials having high magnetic anisotropy energy may not be suitable for memory devices.